A traditional private branch exchange (PBX) is used inside a building and has a base station comprising a switching system connected, through wiring inside the building, to a plurality of remote terminals such as telephone sets, data sets, teletypewriters, computer terminals, and the like. Because of these wired connections, moving or adding remote terminals involves expensive and time consuming wiring or rewiring. These problems can be eliminated by replacing the wiring between the base station and the remote terminals by microwave frequency signals, such as is done currently for single users with cordless phones. However, the use of multiple cordless phones is not practical for use with PBX's because of problems with capacity, adjacent channel interference, and interference from other systems, as described below.
Currently, cordless phones have been allocated only a few channels at about 50 MHz by the FCC. Although adequate for residential use, the capacity is clearly inadequate for buildings using PBX's. Even if more bandwidth were allocated for cordless phones or wireless PBX's (for example, the mobile radio band at 900 MHz), the use of current phone systems would still have problems.
One problem is adjacent channel interference. Present cordless phone systems have fixed remote and base station transmit powers and can operate with signal power variations up to 80 dB at the receiver. Thus, such a system with one base station and multiple remotes can have received signals differing in power by as much as 80 dB at the base station which creates an adjacent channel interference problem.
Another problem is interference from other nearby systems. Signal propagation in buildings varies substantially with the position of the user resulting in a high probability that interfering signals from nearby systems are stronger than the desired signal. Such interference could easily block channels, thereby reducing capacity or abruptly terminating calls.
The above discussion also applies to wireless local area networks (LAN).